The invention relates to the use of a porous, preferably dimensionally stable, material for the extraction of hydrophobic constituents, such as benzene, toluene, xylene and/or chlorinated hydrocarbons, from an aqueous solution, the material containing a hydrophobic substance exhibiting great affinity for the constituents to be extracted from the water.
Such a use, with a porous material being employed, was earlier described in GB-A-1 535 481. The porous material employed in this document consists of mineral carriers which are inert with respect to water, to the hydrophobic substance to be used as extractant, and to the compound to be extracted. Examples of suitable mineral carriers mentioned are pumice, kieselguhr, bauxite, alumina, carbon, or silicates. The particles preferably have a size in the range of 0.1 mm to 5 cm. It is stated that the pore size is critical only to the extent that it should be large enough to permit penetration of the compound to be extracted, of the extraction solvent, and of the regeneration liquid.
In actual practice, using the known materials for the aforementioned extraction process is attended with major drawbacks because the hydrophobic substances absorbed in them are easily displaced by water, especially when a packed bed is employed. In consequence, the water to be extracted or purified is in fact contaminated rather than decontaminated by these substances.
The invention now provides for the use of a porous material with a hydrophobic substance absorbed therein which, a significant increase in the capacity per volume unit notwithstanding, does not give rise to stability problems, not even when utilised on an industrial scale.
The invention consists in that when a porous material of the known type mentioned in the opening paragraph is used, it has a surface which will be wetted more readily by the hydrophobic substance immobilized in the pores of an average diameter in the range of 0.1 to 50 xcexcm than by the aqueous solution, with the proviso that at least 60% of the hydrophobic substance can be extracted from the pores by a liquid wholly made up of the constituents to be extracted.
To determine the average pore diameter advantageous use is made of mercury porosimetry in accordance with ASTM D 4248-83.
According to the invention, preference is given to the use of a porous material at least 85% of which can be extracted from the pores by a liquid wholly composed of the constituents to be extracted.
It is to be considered extremely surprising that by the use of a porous material having a hydrophobic surface as well as an average pore diameter within the given range both the stability and the extraction capacity are so enhanced that now, for the first time, application on an industrial scale has become feasible.
It was found that, in general, optimum results can be obtained when using a material having an average pore diameter in the range of 0.2 to 15 xcexcm.
The hydrophobic immobilized material may be solid matter as well as a liquid. If the immobilized material is a solid, preference is given to a polymer which swells in the hydrophobic substance to be extracted. Examples of polymers which are suitable for use within the framework of the invention include polymethyl(meth)acrylate, styrene-acrylonitrile copolymer, and acrylonitrile-butadiene-styrene copolymer, all of which may be partially cross-linked or not. Preference is given in this case to polystyrene. If the immobilized material is a liquid, use is made of a liquid having the greatest possible affinity for the hydrophobic constituents to be extracted. Needless to say, this liquid should be virtually insoluble in the aqueous solution to be extracted and be so immobilized in the porous material that it cannot flow from the porous structure. Within the framework of the invention preference is given to the use of a liquid in the form of a glycerol ester of one or more, preferably unsaturated, fatty acids. In general, favourable results are attained if the immobilized liquid is an oil, such as palmitic oil, olive oil, peanut oil, paraffinic oil, fish oil such as herring oil, linseed oil, and, in particular, soybean oil and/or castor oil.
Generally, favourable results are attained when using a material of which at least 15 vol. % of the pores is filled with the substance immobilized therein, with optimum results being obtained using a material of which at least 50 vol. % and not more than 95 vol. % of the pores is filled with the substance immobilized therein. Completely filled up pores may cause problems on account of swelling (solid matter) or expansion (liquid), which may lead to the dimensions of the porous material being interfered with or to the immobilized liquid bursting the pores. In the case of porous materials wholly filled up with a liquid hydrophobic substance being used, these difficulties can easily be overcome by mixing the filled material with unfilled (porous) material, so that the generated excess can be absorbed during extraction. Alternatively, when a packed bed is used, the unfilled (porous) material may be deposited at the two ends of the bed as separate boundary layers. When these types of steps are taken, 100 vol. %-filled materials may also be employed.
The porous material preferably is organic in origin. However, it is also possible to use a porous inorganic material, providing it has a hydrophobic surface, e.g., through the application of a coating.
Within the framework of the invention preference is given to the use of natural and synthetic organic materials, the latter materials being preferred for reasons of reproducibility. Examples of synthetic organic materials include porous polymers, more particularly those of which the preparation is disclosed in U.S. Pat. No. 4,247,498.
Examples of polymers deemed more or less suitable for use in the present invention include:
low pressure polyethylene, high pressure polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene-styrene terpolymers, styrene-acrylonitrile copolymers, styrene-butadiene copolymers, poly(4-methyl-pentene-1), and polybutene.
Optimum results were attained by employing polyolefin based polymers, with preference being given to the use of a polypropylene based porous material.
The porous polymer is generally used in the form of grains having an average particle diameter of 0.1 to 10 mm. Alternatively, the polymer may be employed in the granulated form, as well as in the form of membranes, fibres which may be hollow or not, etc. When in the form of a granulate or a powder, the porous polymer may be used in a packed bed, a fluidised bed, or a tank with stirring. Fibres which may be hollow or not are used in the form of fabrics and non-wovens, respectively.
In one embodiment, the porous material maybe obtained by dissolving the polymer in a solvent with heating, cooling the solution to obtain a solidified mass, and the mechanical diminution of the solidified mass if so desired.
Particularly when a glycerol ester of one or more unsaturated fatty acids is employed to extract aromatic compounds from aqueous solutions, preference is given to a porous material obtained by a process such as described in DE-A-32 05 289. By this process a structure may be obtained which has pores of an average diameter in the range of 0.1 to 50 xcexcm. Especially favourable results may be obtained when using polypropylene as porous material and soybean oil/castor oil as hydrophobic substance.
Needless to say, the materials according to the invention are not just suitable for extracting aromatic waste matter from aqueous solutions; extracting useful constituents such as biologically active constituents from usually highly dilute aqueous solutions is also within the bounds of possibility.
Generally, the procedure used to prepare the extracting material according to the invention is as follows: first, 5-90 wt. % of a polymer is dissolved, with heating, at a temperature above the upper critical decomposition temperature Tc in 10-95 wt. % of a mixture of two liquid and miscible compounds A and B, the mixing ratio of A to B being so selected as to give decomposition on cooling, resulting in a polymer-rich and a polymer-poor phase. On further lowering of the temperature this decomposition structure is then fixed prior to the completion of the phase separation, due to vitrification or crystallisation of the polymer, resulting in a porous polymer material filled with the mixture of compounds A and B which is pre-eminently suited to be used within the framework of the invention, either as such or after diminution if so desired.